1. Field of the Invention
The present invention relates to a stereophonic voice recording and playback device for recording a voice signal on a solid-state recording medium such as a semiconductor memory and for reproducing such recorded information as a voice signal. And more particularly, the present invention relates to a solid-state stereophonic voice recording and playback device suitable for use as a stereophonic conference dictating machine.
2. Description of the Related Art
FIGS. 1 and 2 show a recording section and a playback section, respectively, for a conventional voice recording and playback device such as disclosed by Japanese Laid-Open Patent Publication No. 55-58815.
Hereinafter, the configuration and operation of the conventional voice recording and playback device will be described. First, an analog waveform representing a voice signal serves as an analog input signal which is sampled by a sampling circuit 100 at a predetermined sampling rate. The sampled values are supplied to an Analog-to-Digital (A/D) converter 101, where the sampled analog waveform is converted to a digital signal. The digital signal is subsequently supplied to an information compressing circuit 102, where the digital signal data is compressed in accordance with a predetermined compression procedure. The compressed data is then written into and stored in a voice information recording section 111 of a solid-state record section 110. At this time, information to indicate a reconstruction procedure corresponding to the compression procedure used by the information compressing circuit 102 is written to and stored in a reconstruction procedure recording section 112 of the solid-state record section 110.
The solid-state record section 110 further includes a controller 113. To reproduce data, the data stored in the voice information recording section 111 is read out, and then the original data before compression is reconstructed using the controller 113 in accordance with the reconstruction procedure information stored in the reconstruction procedure recording section 112. Thereafter, the reconstructed data is supplied to a Digital-to-Analog (D/A) converter 120, where it is converted to the original analog voice signal, and subsequently it is outputted to a speaker section.
According to the above-described voice recording and playback device, only a monophonic voice signal can be recorded and reproduced. A position from which a voice originates cannot be ascertained, such as is possible with a stereophonic voice recording and playback device. Therefore, a conventional voice recording end playback device as described above cannot be applied to a conference dictating machine which enabled us to recognize a position of each speaker for readily determining who speaks.
Even though it is possible to record and reproduce a stereophonic voice signal by modifying the above voice recording and playback device according to conventional techniques, there arises a problem of a high production cost. Such modification for providing stereophonic recording and playback necessitates each of two circuits 100 and 102 and converters 101 and 120 and at least one solid-state record section 110. According to such a configuration, the required information content is doubled, which disadvantageously decreases the recording and playback time to one-half. That is, the required memory capacity of the solid-state record section 110 is increased twice as much as that of the stereophonic voice recording and playback device for recording and reproducing data providing the same amount of recording time. For the above reasons, when the conventional voice recording and playback device is used for recording and reproducing the stereophonic voice signal, the configuration thereof becomes complicated and thus the production cost is significantly increased.
FIG. 3 shows a conventional method for converting analog signals in two channels to digital signals using a single A/D converter, such as disclosed in Japanese Laid-Open Patent Publication No. 55-17850.
In FIG. 3, the analog signals L and R in the corresponding two channels are inputted to low-pass filters 71 and 71', respectively. The outputs of the low-pass filters 71 and 71' are inputted to sample and hold (S/H) circuits 72 and 72', respectively, where each analog value is sampled and held, at a predetermined sampling rate, and outputted to an analog switch 73. Using the analog switch 73, the sampled signals L and R are alternately outputted to an A/D converter 74, where they are converted to digital signals in alternating order such as L.sub.n, R.sub.n, L.sub.n+1, R.sub.n+1, . . . . That is, the digital signals of L.sub.i and R.sub.i are alternately outputted.
To sample the signals L and R at a frequency of F.sub.s Hz, the S/H circuits 72 and 72' should be operated at the frequency of F.sub.s Hz, and the analog switch 73 should be operated at a frequency of 2.times.F.sub.s Hz. Accordingly, the A/D converter 74 should have a band width of 2.times.F.sub.s Hz, and thus the A/D converter 74 is required to be operated at a high speed with high precision. However, according to this A/D converter 74, the signals L and R are alternately converted, so that samples from the same signal cannot successively be converted. Therefore, a delta-sigma modulation type A/D converter including a delay circuit cannot be used for this purpose even though such a delta-sigma modulation type A/D converter, which employs a primarily digital circuit technique, not requiring a high-precision analog circuit technique in order to achieve high-precision A/D conversion without requiring adjustment, is very useful for an integrated circuit.